Necrons are pretty tough (hard to put down and harder to keep down). If you have means to safely transport them you can use my Tyranids (1200pts approx). They are easy to play; just charge them at the enemy and start biting.

The only downside with my Nid's is that their req'd HQ choice is quite expensive for a 500 point build (about 200 points). Mind you the Necrons have the same problem, their basic troop type is 19 points a model and they need at least two squads of 10 so you might be alright.

_________________"I've got a lantern, a pair of waders and possibly the most fearsome piece of hand artillery in all England - what could possibly go wrong?"